The present invention relates to the field of signal processing, and in particular, to the field of extrema coding signal processing. In extrema coding, an analog signal is encoded, along with broadband noise having a spectrum containing frequencies significantly higher than the highest frequency in the analog signal, as a series of transitions identifying the extrema of the analog signal and the noise. Thus, the extrema, i.e., the maximum and minimum points of the analog signal and the noise are converted to zero crossings of a binary signal. In a preferred implementation, the analog signal and noise are first subject to differentiation, which converts the maximum and minimum points to zero crossings, and then the zero crossings are provided to a broadband infinite clipper, which converts the zero crossing points into defined transitions of a binary signal. The binary signal contains sufficient information to enable substantial reproduction of the analog signal by a filter device, for example, an integrator. The noise signal necessary for extrema coding may either be added to the analog signal or naturally present, and if added, it may be added either before or after differentiation. If added after differentiation, the transitions of the encoded signal technically represent zero crossings of the noise signal in addition to extrema of the analog input signal, but the coding process is essentially the same in either case.
Extrema encoded signals provided to the human ear sound subjectively the same as the original analog signal, even though the time domain waveform of the extrema coded signal is very different than the original analog signal. It is postulated that the human ear essentially performs a filtering or integrating function much as the integrator circuit can be used to recover substantially the analog signal. It also appears that other portions of the human sensory system, i.e., the human eye, also are responsive to extrema encoded signals.
The basic patent for extrema coding is applicant's U.S. Pat. No. 4,545,065. U.S. Pat. No. 4,700,360 discloses an extrema coding digitizing signal processing method and apparatus, wherein an extrema encoded signal is subjected to a digitization process so that it can be transmitted along a transmission medium.
Extrema coding offers several advantages, most notably, improved dynamic range due to the fact that an extrema encoded signal is a binary signal, relative simplicity of circuitry and signal waveform, and the ability to conserve bandwidth by reducing the amount of information necessary to be transmitted. This results from the fact that the human sensory system is most receptive to the location of the extrema or maximum and minimum points of the signal received by the sensory system, and not to the other information present in the signal. However, there must be sufficient background noise having a spectrum including frequencies higher than the highest frequency in the analog input signal, in order that the extrema coding principle can work. If there is insufficient background noise, noise may be added by a suitable noise source. Without adding background noise to the analog input signal, or if there is insufficient analog noise or if the broadband infinite clipper does not have a sufficiently high bandwidth, then the human sensory system will perceive a distorted signal. A distorted signal was observed by, for example, Licklider and Pollack in their experiments on differentiated, integrated and clipped waveforms, as set forth in their paper entitled "Effects of Differentiation, Integration, and Infinite Peak Clipping upon the Intelligibility of Speech," Journal of the Acoustical Society of America, Vol. 20, No. 1, 1948, pp. 42-51. Licklider and Pollack did not recognize the significance of the background noise in the analog signal, and thus never comprehended the principles of extrema coding.
The present invention is thus related generally to the field of electronic signal processing, and particularly to the area of sensor systems, preamplifiers and feature selection systems.
More particularly, the invention can be better understood by applying some of the principles of the theory of perception.
Information in the world around us is of an extraordinarily complex nature. The complexity of the data that we as observers would like to take in is such that analysis of all that could be relevant is not feasible. An intelligent selection process must precede the brain as a tool to take decisions. The more intelligent the selection process, the more effective the final interpretation of the outside world will be.
The present invention can be used to construct such a selection system. The selection mechanism is normally regarded to be an integral part of a biological perception system. It is, therefore, suggested that the method according to the present invention might find application in cases where existing perception mechanisms are impaired or need to be improved for other reasons. As a feature selection mechanism, the invention can accomplish a significant amount of information reduction. This results in the opportunity to utilize the invention in communication systems. The invention may also find application in pattern recognition machines and could be used in processes to realize artificial intelligence.
Broadly, the invention comprises an improvement to the signal processing system known as extrema coding.
A brief description of extrema coding including some of the problems overcome by the new technique now follows.
As described above, extrema coding is a method that applies infinite peak clipping to detect, process and amplify an input waveform. Normally, the clipping process will reduce the information content of the signal in a rather crude manner. Features of information that are usually perceptible will be totally missing in the clipped waveform. The clipping process will give rise to entirely new components referred to as harmonic distortion. This makes clipping in general unsuited for high quality feature detection. Extrema coding relies on the notion that a waveform can be clipped without causing significant degradation when the waveform is in the presence of random noise of a bandwidth of at least a factor greater than that of the signal. Background noise of sufficient energy can usually be found accompanying this signal, guaranteeing practicality of the extrema coding technique. This background noise may not be totally random, but it is sufficient to allow practical extrema coding. If insufficient background noise is present, however, artificially generated noise may be added, as discussed above.
Extrema coding can be described with reference to FIG. 1. Signals such as those normally provided to the human perception system will consist of a superposition of waveforms generated by a number of different sources. Simplified, the combined waveform will comprise a primary most significant waveform, usually called the focal stimulus, accompanied by a number of background variations, the background stimulus. A third waveform, relating to the conditions imposed by the perception mechanism, may be added in the form of wide-band random noise. This third signal can be called the residue stimulus. The three waveforms, combined as shown at the output of summing block 10 in FIG. 1, are provided to a differentiator 20 in a preferred embodiment of an extrema coding system. The differentiator 20 changes the times of occurrence of the minima and maxima (the extrema) of the combined signal into time domain zero crossings. An infinite clipper can be used to detect the zero crossings and change all other input levels to one of two signal levels, a positive or a negative amplitude. These levels are shown by +A or -A in FIG. 1. The broadband infinite clipper is indicated at 30. Due to the fact that the bandwidth of the noise is significantly greater than the bandwidth of the focal stimulus, the clipping circuit will need to be fast enough not only to detect the extrema of the focal stimulus, but also to switch at the transitions of the noise. The clipped signal output is provided to an integrator 40 or to some other low pass filter circuit. At the output, a waveform will appear that may closely resemble the original focal and background stimulus. The input/output characteristic of the extrema coding system largely depends upon the probability density distribution of that portion of the input signal that is considered to be the noise. If the noise is shown to have a Gaussian distribution, the system has been shown to give an exponential, error function like input/output relationship. Despite the obvious degradations during a short time interval (for example up to 10 zero crossings), and the amplitude compression and masking effects, the extrema coding signal processing system has been demonstrated to give little or no subjective degradation when provided with both audio and video analog input signals.
Extrema coding largely relies on the availability of broadband random noise. In the absence of such additive noise, output signals can be strongly degraded. It is the bandwidth of the noise that determines the actual output quality. When signal levels are moderate to high, the background stimuli will often provide sufficient extrema, in which case the added noise or residue noise may not be required. In cases where low level waveforms are provided, the absence of noise will result in a low zero crossing rate causing severe degradation. When no input signal is present, the added noise and especially its bandwidth is most critical. The noise will be clipped resulting in a non-zero output level. In order to provide a zero output level in the absence of an input signal, the noise spectrum theoretically would have to be infinite. It can be calculated that the RMS output voltage will drop with 3 db for each octave of noise bandwidth that is added.
In order to obtain acceptable signal to noise ratios, for instance, for communications systems, this translates into the need for clipping speeds exceeding the 10 MHZ range. With the current state of the art of electronic components and designs techniques, it can be difficult and costly to operate at these speeds. This applies to both the clipper and the noise source.
Further, extrema coding signal processing techniques are limited by constraints imposed by noise generator designs. The signal response of the extrema coding signal processing circuit is determined largely by the probability density distribution of the noise. The probability density distribution will determine the dynamic range of input levels that can be handled. As a perception stimulation system, extrema coding signal processing techniques have to function over a range of over 100 db for acoustic signals. Limitations were imposed partially by the sensitivity and overdrive capabilities of the clipper, but mainly by the dynamic range of noise levels. Circuits to increase dynamic range, for instance expanders, exist, but impose additional gain/bandwidth limitations.